Impatiens Downy Mildew
Janna Beckerman, Associate Professor, Department of Botany & Plant Pathology, Purdue University
Impatiens downy mildew is a new threat to flowering impatiens in greenhouses and landscape. Impatiens walleriana (all flowering impatiens, including doubles and minis) and any I. walleriana hybrids (I. walleriana X auricoma and I. niamniamensis X walleriana) are considered susceptible, as is garden balsam, I. balsamina. Although reported on native wild impatiens, I. capensis , commonly known as spotted jewelweed, this has not been confirmed. Finally, I. glandulifera , commonly called policeman’s helmet or Himalayan balsam, is also a reported host, but should be avoided due its potential to be an invasive species.
Early symptoms of impatiens downy mildew are difficult to detect: Leaves may be slightly yellow (chlorotic) with stippling , and may mimic nutritional deficiency or spider mite damage (Fig. 1). Leaves droop and the plants take on an “unthrifty” appearance. Under cool, wet humid conditions, white-colored sporulation may be visible on the undersides of the leaves (Fig. 2), but it is important to stress that sporulation may not always be visible, especially if conditions are warm and dry (Fig. 3). As the disease progresses, plants appear stunted, and foliage drops—resulting in green, leafless stems (Fig. 4).
Downy mildew is caused by the water mold Plasmopara obduscens, and is closely related to Phytophthora and Pythium diseases. As the name ‘water mold’ suggests, moisture is a key part to this pathogen’s life cycle, and is necessary for it to sporulate and cause new infections. Thus, in any place where the leaves stay wet for extended periods of time (e.g., plants that are grown tightly together, or in heavily shaded locations, or where watered extensively) creates conditions that foster this disease if the pathogen is present.
Due to the necessity of multiple applications of fungicide to manage this disease, fungicide use is not recommended for landscape plants. Sanitation is the best management strategy for this disease, and all infected impatiens should be removed and destroyed to minimize the amount of overwintering inoculum (called oospores), which can potentially start a new epidemic next year if impatiens are replanted in the area. To date, we do not know the overwintering potentially of this pathogen in zone 5, but numerous reports exist of it persisting through zone 6. We are currently growing some plants on the Purdue campus to see if it can overwinter here.
Few plants provide the flowering impact in the shade like flowering impatiens, but growers with a history of problems should consider other crops. Alternative annuals that do not get this disease include New Guinea impatiens, begonia, caladiums, coleus, cyclamen, fuchsia, lobelia, perilla, torenia, and viola/pansies.
Click image to enlarge
Figure 1. Early symptoms
Figure 2. White-colored sporulation on underside of leaves
Figure 3. Sporulation not visible under warm, dry conditions
Figure 4. Stunted plants and leafless stems